The present invention relates to microstructure fabrication, and in particular, to a dry-release method for sacrificial layer microstructure fabrication.
The use of microstructures in integrated circuits and other micromechanical structures is becoming increasingly common. Microstructures are currently used as wiring structures in integrated circuits, as parts of microelectronic components, and as micromechanical structures such as movable joints, levers, gears, sliders, and springs.
In order to build microstructures, a sacrificial layer must first be deposited upon a substrate, and a structural component or layer deposited upon the sacrificial layer. The sacrificial layer is then removed, leaving a substrate with a structural component attached to, but spaced from the substrate. Generally, two types of methods are currently used to remove the sacrificial layer. One type is the wet-release method, and the other type is the dry-release method. However, problems exist with each method.
In wet-release methods, during sacrificial layer removal, capillary forces develop between the structural layer and the substrate which tend to deflect the structural layer towards the substrate, causing it to undesirably adhere thereto. In a typical wet-release method, illustrated in part in FIGS. 1(a)-1(c), an etching solution or etchant is applied between the substrate 2 and structural layer 4 to remove the sacrificial layer. The sacrificial layer is then rinsed away by a rinsing solution 6, as shown in FIG. 1(a). As the etching and rinsing solutions 6 evaporate from the small space between the structural layer 4 and substrate 2, strong capillary forces between the two are created. As the volume of liquid trapped beneath the structural layer 4 decreases by evaporation, the capillary forces become stronger. As a result of these forces, the structural layer 4 begins to deflect toward the surface of the substrate 2, as shown in FIG. 1(b). If the capillary forces are strong enough and the structural layer 4 weak enough, the structural layer 4 contacts the substrate 10. At that point, intersolid forces are the strongest, and the structural layer 4 may be permanently pinned to the substrate 2. Therefore, a need exists for a method that prevents the destructive effects of capillary forces associated with wet etching techniques.
To overcome the destructive effects of the capillary forces, dry-release methods have been developed. While the dry-release methods for removing sacrificial layers are not troubled by capillary forces, they have their own distinct drawbacks. For example, the dry-release method of Saiki et al, U.S. Pat. No. 3,846,166, requires deposition of a structural layer on a sacrificial resin layer. However, because the resin layer melts or degrades at low temperatures, e.g. 300.degree.-400.degree. C., many desirable materials for the structural layer which require higher deposition temperatures cannot be deposited upon the fragile sacrificial resin layer. For example, polysilicon, a preferred material for microstructures, requires a temperature of about 600.degree. C. for deposition.
Similarly, Bly et al, U.S. Pat. No. 4,849,070, teaches a dry-release method for removing a sacrificial layer in which the structural layer is built on top of a solid layer that can later be sublimed to free the structural layer. However, the choice of structural layer materials is again limited to lower temperature materials, as higher temperature materials cause the sublimable layer to disappear before deposition thereon. In addition, Bly et al teaches the production of permanent posts which support the structural layer above the substrate, and form part of the finished microstructure. However, the need exists in many applications for microstructures absent such permanent posts.
Other dry-release methods known in the art include a liquid freezing and sublimation method and a photoresist refill and plasma ashing method. These methods, too, have drawbacks. The liquid freezing and sublimation method is unreliable, and, in part, uncontrollable. The freezing of the liquid between the structural layer and substrate can result in an increase in volume which leads to fracture of the microstructure. The photoresist refill and plasma ashing method is difficult to perform, as careful, time consuming and costly mixing of solutions is required.
Accordingly, there is a need in the art for a novel dry-release method for sacrificial layer microstructure fabrication which avoids the effects of capillary forces created by the wet etching of a sacrificial layer, uses a sacrificial layer upon which a wide variety of structural layers may be deposited at higher temperatures, produces a microstructure without permanent posts, and is simpler and more reliable than the methods currently in existence.